This invention relates to systems which utilize received signals from satellite positioning systems (SPS) to locate themselves or to determine time-of-day.
SPS receivers such as GPS (Global Positioning System) receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of satellites such as GPS (or NAVSTAR) satellites. These satellites transmit, as part of their satellite data message, both satellite positioning data as well as data on clock timing, so-called "ephemeris" data. In addition they transmit time-of-week (TOW) information that allows the receiver to determine unambiguously local time. Each received GPS signal (in C/A mode) is constructed from a high rate (1.023 MHz) repetitive pseudorandom (PN) pattern of 1023 symbols, commonly called "chips." Further imposed on this pattern is low rate data at a 50 Hz rate. This data is the source of the above mentioned time-of-week information. The process of searching for and acquiring GPS signals, reading the ephemeris data and other data for a multiplicity of satellites and computing the location of the receiver (and accurate time-of day) from this data is time consuming, often requiring several minutes of time. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in micro-miniaturized portable applications.
In addition, in many situations, where there is blockage of the satellite signals, the received signal level from the GPS satellites is too low to demodulate and read the satellite data signals without error. Such situations may arise in personal tracking and other highly mobile applications. Under these situations it is possible for a receiver to still acquire and track the GPS signals. However, performing location and unambiguous time measurement without such data requires alternative methods.
Tracking the GPS signals without reading the data messages may result in 1 millisecond ambiguities in time, as explained below. Such ambiguities are normally resolved in a conventional GPS receiver by reading the satellite data message, as previously described. At very low received signal levels, the pseudorandom pattern may be tracked, or otherwise used to provide ambiguous system timing by processing many repetitions of this signal (e.g. 1000 repetitions over 1 second). However, unless the signal-to-noise ratio measured over one data period (20 milliseconds) is above about 12 dB, there will be many errors present when attempting to demodulate this signal. The current invention provides an alternative approach for resolving ambiguities in time when such reading is impossible or impractical.